Only pass non-None sortby to PrettyTable.get_string

Fixes bug 1256883

Older versions of prettytable are not tolerant to sortby
being provided as an explicit kwarg set to None.

We avoid this issue by leaving the kwarg unspecified when
sorting is not required.

Change-Id: I4ef7bc73a820a63d790a23a0f4e64a1077274d8a